Method for neutralizing waste sulfuric acid by adding a silicate

ABSTRACT

It has been shown that magnesiumsilicates, particularly Mg-rich olivine, is very advantageous for neutralizing waste sulfuric acid. A silicagel and a Fe/Mg-containing liquid are formed. The experiments show that a distinct separation of the silicagel from the Fe-containing liquid is possible in case the neutralization process is performed in an inert atmosphere. After separation of the SiO 2  -rich gel a Fe-V precipitate may be formed by means of aeration. The SiO 2  gel may be rinsed well, so that an acceptable product is formed that e.g. is useful in the concrete industry. The liquids formed are MgSO 4  rich. They may simply be drained into sea-water.

The invention relates to a method for neutralizing waste sulfuric acidby adding a salt of a weaker acid than sulfuric acid.

As appears from "Ullmann's Enzyklopadie der chemischen Technologie" 18,579 (1982), it was common practice up to now to drain waste aciddirectly into the sea or into rivers. Said draining causes anunacceptable environmental effect on the surface waters. If from thewaste sulfuric acid gypsum is formed by reacting it with calciumcarbonate, the waste acid problem is only transformed, since in thatcase one obtains considerable quantities of gypsum for which aninsufficient number of applications is available, particularly in viewof the impurities present in that type of gypsum. (Consequently,according to the law, said gypsum is still considered a chemical waste).

It was found now, that the waste acid problem may be solved in anenvironmentally acceptable way by adding magnesium silicate to saidwaste sulfuric acid. Upon reaction with sulfuric acid two separate oreasily separable phases are formed, the first of which consists of anaqueous solution of the sulphate ions and the magnesium metal ionsderived from the salt and the second of which contains an SiO₂-containing precipitate, apart from the magnesium silicate thatoptionally is used in excess.

From a practical point of view magnesium silicate will be used, thatreacts sufficiently quickly with sulfuric acid.

Magnesium silicates are e.g.: forsterite, olivine, enstatite andchrysolite.

Subsequently, the solution of sulfate ions and metal ions may be drainedinto surface water, or it can be used for an industrial application.This holds also for the precipitate obtained.

Magnesium is the second most important cation in sea-water andconsequently there are no serious objections to the draining thereof. Asimilar argument holds for the sulfate ion, which is the second mostimportant anion. Since further in case of using magnesium silicate asilica gel will be formed below the magnesium sulfate solution, andsilica gel may be used for many purposes in the building arts, the useof magnesium silicate is very useful.

The reaction that occurs is illustrated by the following equation:

    2H.sub.2 SO.sub.4 +Mg.sub.2 SiO.sub.4 →H.sub.4 SiO.sub.4.aq+2SO.sub.4.sup.2- +2Mg.sup.2+

Said reaction occurs completely even at room temperature (18°-30° C.).The reaction rate depends on the surface area available and on thetemperature. In a series of experiments it appears that at a temperatureof 50° C. the reaction time is about 24 hours and at 85° C. it is justseveral hours. Literature data show that magnesium-rich olivine is themost advantageous of all magnesium silicates. It may be expected that ifother magnesium silicates would be used, the reaction time would be atleast ten times higher.

Olivine occurs in high concentrations in nature in stones that can beobtained easily. In a preferred embodiment of the method according tothe invention particularly as regards using it on an commercial scale,olivine is added to the sulfuric acid. Indeed olivine contains apartfrom magnesium also small quantities of other metals, like nickel, butsaid metals may be extracted easily by means of known methods. Furtheriron is present in the solution. Said iron may be advantageous, sinceupon precipitation of iron hydroxide heavy metals may be precipitatedsimultaneously and subsequently they can be separated easily.

EXAMPLES

After having observed in a series of experiments that in order toachieve a fast reaction of the liquid olivine blend it has to be stirredcontinuously, and that a smaller grain size (higher specific surface)and a higher temperature increase the reaction rate, some experimentswith waste sulfuric acid were carried out.

SERIES I

350 g. olivine, having the following a composition: SiO₂ 40.7 weightpercent, MgO 50 weight percent, FeO 8.9 weight percent, NiO 0.4 weightpercent, were added to 300 ml waste sulfuric acid (s.g. 1.29) having thefollowing composition: H₂ SO₄ 29 weight percent, Fe 1.8 weight percent,Mg 0.5 weight percent, Al 0.4 weight percent, Ti 0.5 weight percent, Cr0.017 weight percent, V 0.050 weight percent. The amount of olivine isthe quintuple of the amount required for a complete neutralization ofthe sulfuric acid. The grain size of the olivine varied from 0.15 to 0.5mm.

At 25° C. after 45 hours a pH of 3.5 was reached. At 50° C. it took 7hours and at 80° C. 11/2 hour. After the reaction the mixture was notstirred for several hours. In all cases three layers were formed, thelower layer contained substantially only olivine. The middle layer was athick layer of a brown gel and the upper layer was a thin layer of aclear, somewhat green coloured liquid. The analysis of the clear liquidshowed that it contained 35 weight percent MgSO₄, while Fe and V couldnot be detected; the Cr percentage was 0.02 weight percent.

The gel contained 55 weight percent H₂ O; 10 weight percent SiO₂ ; 5weight percent Fe₂ O₃ ; 30 weight percent MgSO₄ ; 0.07 weight percent Vand 0.02 weight percent Cr. After repeated rinsing of the gel the majorpart of the MgSO₄ could be removed. Then, the gel contained about 75weight percent H₂ O; 12 weight percent SiO₂ ; 7 weight percent Fe₂ O₃ ;6 weight percent MgSO₄ ; 0.1 weight percent V and 0.12 weight percentCr.

SERIES II

This series is identical to series I but the system was kept in areducing condition by maintaining a nitrogen atmosphere above theliquid-olivine blend.

There was no difference in reaction time. After having stopped stirringthe system divided itself again into three layers. The lower layer wasformed by olivine. The second layer consisted of a transparent to whitegel above which the clear green liquid was present.

The composition of the gel was 60 weight percent H₂ O, 10 weight percentSiO₂, 5 weight percent FeSO₄ en 30 weight percent MgSO₄ with 0.03 weightpercent V and 0.02 weight percent Cr.

After rinsing the composition of the gel was 85 weight percent H₂ O, 12weight percent SiO₂, 1 weight percent FeSO₄ and 1 weight percent MgSO₄.

After centrifuging the composition of the gel was 60 weight percent H₂O, 38 weight percent SiO₂, 1 weight percent FeSO₄ and 1 weight percentMgSO₄.

The green liquid was oxidized at pH 3.5 by means of H₂ O₂.

Immediately a brown precipitate was formed having the composition: 70weight percent FeOOH, 10 weight percent MgSO₄, 20 weight percent H₂ O,0.7 weight percent V and 0.03 weight percent Cr.

The remaining liquid contained 35 weight percent MgSO₄ and 0.02 weightpercent Cr.

I claim:
 1. In a method for reacting waste sulfuric acid with themagnesium silicate of olivine; the improvement comprising the stepsof:adding olivine to dilute aqueous waste sulfuric acid, in an amount atleast several times in excess of that required fully to react with saidacid; effecting said reaction in an inert nitrogen atmosphere; agitatingthe resultant reaction mixture; allowing said reaction mixture tosettle, thereby to obtain a three-phase system consisting of a bottomphase comprising excessive olivine, a middle phase comprising silicagel, and a top phase comprising an aqueous magnesium sulfate solution.